feat: extract MessageClipboard for plaintext and quote formatting

Centralize pasteboard writes and composer quote blocks so message
actions stay testable without SwiftUI.

bitchat/Services/MessageClipboard.swift

@@ -0,0 +1,57 @@
+//
+// MessageClipboard.swift
+// bitchat
+//
+// This is free and unencumbered software released into the public domain.
+// For more information, see <https://unlicense.org>
+//
+
+import Foundation
+#if os(iOS)
+import UIKit
+#elseif os(macOS)
+import AppKit
+#endif
+
+/// Clipboard helpers for message actions. Kept out of the view so unit tests
+/// can assert quote formatting without standing up SwiftUI.
+enum MessageClipboard {
+    /// Plain message body for the pasteboard.
+    static func copyPlaintext(_ content: String) {
+        #if os(iOS)
+        UIPasteboard.general.string = content
+        #elseif os(macOS)
+        let pasteboard = NSPasteboard.general
+        pasteboard.clearContents()
+        pasteboard.setString(content, forType: .string)
+        #endif
+    }
+
+    /// Builds a composer-ready quote block (`> line` per line, trailing blank).
+    static func quoteForComposer(_ content: String, sender: String?) -> String {
+        let body = content.trimmingCharacters(in: .whitespacesAndNewlines)
+        guard !body.isEmpty else { return "" }
+        let header: String
+        if let sender, !sender.isEmpty, sender != "system" {
+            header = "> @\(sender):\n"
+        } else {
+            header = ""
+        }
+        let quoted = body
+            .split(separator: "\n", omittingEmptySubsequences: false)
+            .map { "> \($0)" }
+            .joined(separator: "\n")
+        return header + quoted + "\n\n"
+    }
+
+    /// Appends a quote to an existing composer draft without wiping it.
+    static func appendQuote(to draft: String, content: String, sender: String?) -> String {
+        let quote = quoteForComposer(content, sender: sender)
+        guard !quote.isEmpty else { return draft }
+        if draft.isEmpty { return quote }
+        if draft.hasSuffix("\n") || draft.hasSuffix(" ") {
+            return draft + quote
+        }
+        return draft + "\n" + quote
+    }
+}